In Ardour, you work in two main windows: the Editor and the Mixer.
To switch between those windows, use the buttons (in the upper right), the shortcut M, or the menu Window > Editor (or Mixer) > Show. Both windows can be visible at the same time (eg. for a multi-monitor setup) using Detach in the same menu.
The Editor window includes the editor track canvas where you can arrange audio and MIDI data along a timeline. This is the window you will be in while editing and arranging a project. The window has a general "horizontal" sense to it: the timeline flows from left to right, the playhead showing the current position in the session moves from left to right — the window really represents time in a fairly literal way.
The Mixer window on the other hand represents signal flow and is the window you will probably be using most when mixing a session. It includes channel strips for each track and bus in your session. It has a general "vertical" sense to it: signals flow from the top of each channel strip through the processing elements in the strip to reach the output listed at the bottom.
It is possible to show a single channel strip in the editor window, and some people find this enough to work on mixing without actually opening the mixer window. Most of the time though, you will want both of these windows at various stages of a session's lifetime — sometimes you'll be focused on editing, sometimes on mixing and possibly some of the time on both.
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